1. Field of the Invention
This invention relates generally to methods and equipment for testing electronic devices, and more particularly to techniques for accurately analyzing the spectra of non-coherently sampled test signals.
2. Description of Related Art Including Information Disclosed Under 37 C.F.R. 1.97 and 1.98
Test programs for automatic test equipment (ATE) commonly measure the spectra of signals sampled from devices under test (DUTs). FIG. 1 shows a conventional test scenario. An automatic test system 110 includes a host computer 112 that runs a test program. The test program activates a signal source 114 to apply a stimulus, which generally comprises one or more test tones, to the input of a DUT 120. A capture instrument 116 samples an output signal from the DUT 120 as it responds to the stimulus. The output signal from the DUT 120 includes the test tones, as well as noise and distortion. Tester software computes a Discrete Fourier Transform (DFT) of the output signal to produce a power spectrum. The magnitudes of the test tones are reported as the power levels of the DFT at frequencies corresponding to the test tones. Noise and distortion can be reported as the power levels of the DFT at all other frequencies.
As is known, “leakage” error manifests itself in the DFT of a sampled test signal whenever the sample clock is not “coherent” with the tones of the input signal. Leakage is the mathematical consequence of performing a DFT on truncated tones—i.e., tones that do not complete an integer number of cycles within the sample window. Leakage can be observed as an erroneous broadening of spectral lines, a creation of false peaks and troughs (lobes), and a general elevation of the power spectrum's noise floor. A sample clock is “coherent” with a test signal if its period multiplied by the number of captured samples is a precise integer multiple of the period of each tone in the test signal.
An important characteristic of ATE is the ability to measure accurately noise and distortion from devices under test. Leakage directly impairs a test system's ability to measure noise and distortion by elevating the apparent noise floor of a DFT to a point where the noise and distortion from the device can no longer be observed. Leakage also impairs the test system's ability to measure the magnitudes of the test tones themselves, because leakage induced from one test tone affects the magnitude of that test tone, as well as the magnitudes of all the other test tones.
Several techniques have been used to reduce leakage. One technique is to multiply the sampled sequence by a windowing function that gradually tapers the sampled sequence to zero at its endpoints. The windowing function forces the windowed sequence to be periodic, and therefore coherent, within the sample window. It accomplishes this, however, at the expense of distorting the spectrum of the sampled signal and increasing the number of samples that must be captured, and therefore increasing the test time.
Another technique is to mathematically convert the actual sampling rate to a rate that is coherent with the tones in the sampled signal. Sample rate conversion works by interpolating between actual points sampled at one rate to mathematically construct a series of points that appear to have been sampled at another rate. Although sample rate conversion can reduce leakage, it requires significant computation time and its accuracy can suffer from interpolation errors.
Still another technique is to vary the rate of the actual sampling clock to ensure coherency. For example, in FIG. 1, values K1 and K2 of dividers 124 and 126 are programmed to ensure that the width of the sampling window precisely equals an integer multiple of the period of every tone in the sampled signal. This technique is effective but requires expensive and complex hardware. It is particularly expensive when a tester includes a large number of sample clocks, as is often the case.
Manufacturers of automatic test equipment seek to improve their products by providing less costly solutions to conventional testing problems. Great benefits can be gleaned from increasing tester performance while decreasing tester cost and complexity. To this end, there is a strong incentive for inexpensively and conveniently reducing leakage to allow test systems to accurately measure the spectra of captured test signals.